In recent years, multifunctional and high-performance small semiconductor devices have been rapidly developed, which are typified by mobile devices such as a mobile computer and a mobile phone. With the development of these semiconductor devices, a transistor-type semiconductor memory element (hereinafter also referred to as a “memory transistor”) has attracted attention as a memory included in the semiconductor devices.
As one of the semiconductor memory elements, the market of nonvolatile memories has been growing, to which data can be electrically rewritten and stored after the power supply is shut off. The nonvolatile memories have a structure similar to MOS transistors and are characterized in that a region capable of storing charge for a long time is provided over a channel forming region. In a floating gate nonvolatile memory, charge is injected into a charge accumulation layer (floating gate) through a tunnel insulating film over a channel forming region and stored. In a MONOS (metal-oxide-nitride-oxide semiconductor) nonvolatile memory, charge is trapped in a charge accumulation layer or silicon cluster is used as a charge carrier.
In a memory transistor included in such a nonvolatile memory, a tunnel insulating film formed between a semiconductor layer and a charge accumulation layer is required to be as thin as possible. This is because the tunnel insulating film is required to be thin enough that a tunnel current flows therethrough.
Transistors formed over the same substrate as a memory transistor have different configurations depending on the circuits including the transistors. For example, since a transistor provided in a logic circuit is required to operate at high speed, it is necessary to make its gate insulating film thin. On the other hand, a transistor provided in a circuit for controlling (writing, erasing, reading, and the like) a memory transistor is applied with a voltage as high as that applied to the memory transistor; therefore, it is necessary to make its gate insulating film thick in order to prevent damage to the transistor.
In Patent Document 1, a pixel portion, a driver circuit portion, and a memory portion are formed over the same substrate, and the thickness of a gate insulating film of the memory transistor is made to be smaller than those of gate insulating films of the pixel portion and the driver circuit portion.    [Patent Document 1] Japanese Published Patent Application No. 2000-356788